1. Field of the Invention
The present invention relates to semiconductor integrated circuits for digital-to-analogue or analogue-to-digital conversion, particularly suitable for use in portable telephones, automobile telephones, cordless telephones, image processors, etc.
2. Description of the Related Art
FIG. 10 shows the construction of a conventional digital-to-analogue (D/A) converting system. A D/A converter (DAC) 101 converts n-bit digital signals D1 to Dn into an analogue current signal, and then outputs it. A resistance 102 has the function of converting the current signal into a voltage signal. Between both terminals of the resistance 102, an analogue voltage appears in accordance with the analogue current output by the D/A converter 101. A low pass filter (LPF) 103 filters the analogue voltage signal generated by the resistance 102, to pass only its low frequency components. The filter 103 thereby outputs an analogue voltage signal.
An electronic volume 107 comprises a reversal operational amplifier including an operational amplifier 106, a fixed resistance 104, and a variable resistance 105. The ratio of the fixed resistance 104 to the variable resistance 105 determines the amplification factor of the electronic volume 107. For controlling dispersion in manufacturing process, the output level of each such D/A converting system must be regulated before shipment as products, by changing the value of the variable resistance 105 in the electronic volume 107.
The electronic volume 107 is thus necessary for controlling dispersion in manufacturing process of such D/A converting systems. But, it requires a large area. More specifically, since the electronic volume 107 includes the operational amplifier 106, it cannot but occupy a large area on the semiconductor chip. This causes an increase in size of the whole D/A converting system.
Such D/A converting systems are used in portable telephones or the like, and their high speed operation has been required recently. The above-described low pass filter 103 is an RC type low pass filter with its input and output by voltage, and includes about five operational amplifiers in general. Conventionally, these D/A converting systems were operated at a relatively low speed, so the current consumption of each operational amplifier in the low pass filter 103 was little. In recent years, however, because of the demand for high speed operation of these D/A converting systems, the current consumption of each operational amplifier in the low pass filter 103 has increased to the extent of 2 to 3 mA. In addition, the current consumption of the operational amplifier 106 in the electronic volume 107 is also much. Consequently, the power consumption of the D/A converting system becomes much. This causes short battery lives in portable telephones or the like.
Besides, conventionally, discrete cells of an existing D/A converter 101, an existing low pass filter 103, and an existing electronic volume 107 which are designed individually, are combined into one semiconductor chip, and a resistance 102 is connected as an external resistance to the semiconductor chip. Such an exterior type resistance 102 can bring about a highly accurate fixed resistance, but causes an increase in size of the whole D/A converting system.
FIG. 11 shows the construction of a conventional analogue-to-digital (A/D) converting system. A low pass filter (LPF) 111 is an RC type low pass filter with its input and output by voltage, like the above-described low pass filter 103. The filter 111 filters an analogue voltage signal AN to pass only its low frequency components. The filter 111 thereby outputs a filtered analogue voltage signal. An electronic volume 116 includes an operational amplifier 114, a fixed resistance 112, and a variable resistance 113, like the above-described electronic volume 107. The level of the analogue voltage signal filtered by the low pass filter 111 can be controlled by changing the value of the variable resistance 113. An A/D converter (ADC) 115 converts the analogue voltage signal whose level has been adjusted by the electronic volume 116, into n-bit digital signals D1 to Dn.
Also in this A/D converting system, like the case of the above-described D/A converting system (see FIG. 10), the electronic volume 116 is necessary for controlling dispersion in manufacturing process of such A/D converting systems, but the electronic volume 116 requires a large area, and thus causes an increase in size of the whole A/D converting system.
Besides, also in this A/D converting system, either of the current consumption of each operational amplifier in the low pass filter 111 and the current consumption of the operational amplifier 114 in the electronic volume 116 is much, and it causes short battery lives in a portable telephone or the like using this A/D converting system.
As described above, in either of the conventional D/A and A/D converting systems, the size of the whole system becomes large because the electronic volume with the operational amplifier occupies a large area.
Besides, since either of the current consumption of each operational amplifier in the low pass filter and the current consumption of the operational amplifier in the electronic volume is much, it causes short battery lives in a portable telephone or the like using such a D/A or A/D converting system.
Furthermore, in the D/A converting system (see FIG. 10), the D/A converter 101, the low pass filter 103, and the electronic volume 107 are provided within one semiconductor chip, and the resistance 102 is connected as an external resistance to the semiconductor chip. This causes an increase in size of the whole D/A converting system.